1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method useful for image processing when a luminance reproduction range of an image output device is wider than the luminance reproduction range of an image input device.
2. Description of the Related Art
In recent years, according to widespread use of image-managing devices such as digital cameras, display units, and printers, color image data is exchanged between different devices. Since the characteristics of the devices are different, sometimes the reproduced color or brightness may be different.
This is because color representation methods (e.g., RGB and CMYK) and luminance ranges that can be reproduced by the devices are different. Thus, in order to obtain a good color reproduction match between different devices, it is necessary to perform appropriate color conversion processing in a device-independent color space (e.g., CIEXYZ and CIELAB).
As a color matching technique used for matching colors of the devices, color management system (CMS) is proposed. The CMS in general uses a profile of device characteristics. For example, a profile of a printer is generated based on a color chart. The color chart includes a predetermined patch and is printed by the printer. Then, the printed color chart is measured by a measuring device, and the profile is generated according to the measurement result.
The generated profile includes a relation between a device-dependent color space and a device-independent color space. Then, by correcting color of an image using the generated profile, the color matching of the image is performed. Thus, in order to achieve good color conversion processing, an appropriate profile is to be used.
In outputting an image captured by a digital camera using a printer, a color signal value (e.g., RGB) of the image is converted into a color signal value (e.g., CIEXYZ or CIELAB) of a device-independent color space based on a digital camera profile.
Generally, a color gamut of a digital camera is wider than that of a printer. Thus, a color of an image outside the color gamut of the printer, which cannot be reproduced by the printer, is mapped to a color in the printer color gamut, which can be reproduced by the printer. Such mapping method includes absolute color matching and relative color matching. According to the absolute color matching, the color in the printer color gamut is maintained so that faithful reproduction of the input image is realized. According to the relative color matching, the conversion is performed so that the color tone of the output white color matches the color tone of the input white color. In this way, the impression of the input image and the output image becomes similar.
Then, according to the printer profile, a color signal value in a device-independent color space is converted into a color signal value (e.g., CMYK) in a printer-dependent color space. According to such processing, an image captured by a digital camera is converted into an image that can be processed by a printer, and then the image is output by the printer.
Further, in recent years, according to the progress of devices, demand for accurate reproduction of an object in an image with respect to texture such as luster as well as color is increasing. For example, U.S. Pat. No. 6,181,384 discusses a technique that corrects an image of an input video signal based on a reflected light model and generates metallic luster.
However, if the conventional color image matching technique is used, a color image with luster may not be appropriately reproduced when image processing is performed among a plurality of devices.
First, the reason why the conventional color image matching technique is not useful in appropriately reproducing luster of an image will be described.
Generally, luster of an object greatly depends on bidirectional reflectance distribution function (BRDF). The BRDF indicates reflectance with respect to a combination of an incident light angle and a reflected light angle of an object. When the object is a low luster object, specular reflection component of the BRDF will be similar regardless of the incident light angle and the reflected light angle. However, when the object is a high luster object, the specular reflection component will be greater.
FIG. 1A illustrates the BRDF of a low luster object. FIG. 1B illustrates the BRDF of a high luster object. As illustrated in FIGS. 1A and 1B, the specular reflection component of the high luster object is great. Thus, a great luminance difference is generated between the high luster region and other regions in the image.
Thus, for example, if an image of an object with luster is captured by a digital camera and displayed on a display, the luster of the object is not fully reproduced on the display. In other words, if an image of an object with luster is captured by a digital camera, highlight clipping of the luster region occurs, and the luster of the object is reduced when it is reproduced. Thus, according to the conventional color matching technique, even if such an image is displayed on a bright display, the luster of the object is not fully reproduced.
This mechanism will be described in detail with reference to FIGS. 2A and 2B. FIGS. 2A and 2B illustrate reproduction methods employing the conventional color image matching technique. The illustrations in FIGS. 2A and 2B include a luminance range 1001 of an object with luster, a luminance range 1002 of an image input apparatus such as a digital camera, and a luminance range 1003 of an image output apparatus such as a display.
In FIGS. 2A and 2B, the luminance range of the image input apparatus is narrow compared with the luminance range of the image output apparatus. Further, the maximum luminance value of the image input apparatus is lower than that of the image output apparatus.
FIG. 2A illustrates an example of absolute color matching and FIG. 2B illustrates an example of relative color matching. As illustrated in FIG. 2A, according to the absolute color matching where the luminance range 1002 of the image input apparatus is maintained, the luminance is displayed according to the brightness limited by the image capturing regardless of the luminance range 1003 of the image output apparatus. As a result, the luminance range 1001 of the object cannot be satisfactorily reproduced.
Further, as illustrated in FIG. 2B, according to the relative color matching where gradation is reproduced according to the luminance range of the device, since the image is generally converted into a brighter image according to the luminance range 1003 of the image output apparatus, the color matching accuracy with respect to the gradation region other than the luster region is poor.
Further, with respect to the image acquired by the image input apparatus, the gradation in the luster region is not properly reproduced (highlight clipping occurs). Thus, according to the absolute and relative color matching illustrated in FIGS. 2A and 2B, gradation of the luster region of the output image is not accurately reproduced.